[Downsampling] Move force merge out of the downsampling API and into the downsample ILM action.

[gmarouli]

Currently the downsampling API is responsible for refreshing and force merging the downsampled index. This means that a downsampled index is always force merged even if this is not something the user would like to have.

We would like to give the user more control by:

• Removing force merge from the downsampling API.
• Add a configurable force merge step in the downsampling action. It would be enabled by default to match the current behaviour, but a user will have the freedom to change it if they wish.

For DLM, force merging a downsampled index will not happen automatically anymore, but we can still apply tail merging.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)